The invention relates to Brillouin ring laser gyro.
More particularly, the invention relates to a Brillouin ring laser gyro of the type comprising a fiber ring resonator, which has a sequence of resonance frequencies, neighboring resonance frequencies differring by a "free spectral range" and means for generating two pump laser waves propagating in said fiber ring resonator in opposite directions, the frequencies of said pump laser waves coinciding with resonance frequencies of said fiber ring resonator and differing by an integral multiple of said free spectral range, whereby two opposite-direction Brillouin ring lasers of different frequencies are excited in said fiber ring resonator (18) by stimulated Brillouin scattering.
A ring laser gyro makes use of the Sagnac effect to measure angular rates relative to inertial space. Clockwise and counter-clockwise propagating laser waves are generated in a ring resonator. The frequency of the generated laser waves depends on the resonance frequency and thus on the optical path length of one passage of the ring resonator. If such a ring resonator is rotated relative to inertial space about an axis normal to its plane, the optical path lengths for the clockwise and counter-clockwise waves become different due to the Sagnac effect. Correspondingly, also the frequencies of the "clockwise" ring laser and of the "counter-clockwise" ring laser become different. If the laser waves propagating clockwise and the laser waves propagating counter-clockwise are superposed, a beat frequency will result, which is proportional to the angular rate. Typical ring laser gyros are He-Ne-gas laser with a helium-neon gas mixture as laser medium and with a triangular ring resonator defined by mirrors. Such ring laser gyros exhibit a "lock-in" in the range of small angular rates. In this case, due to back-scattering of light of the "clockwise" ring laser into the "counter-clockwise" ring laser and vice versa, both lasers end up in operating at a common frequency. There is no beat frequency proportional to angular rate (Frederick Aronowitz "The Laser Gyro").
Brillouin ring laser gyros are known. With such ring laser gyros, the ring resonator consists of a fiber ring. Light from a pump laser with sufficient energy is coupled into this fiber ring resonator once clockwise and once counter-clockwise. Acoustic waves are stimulated in the fiber ring resonator by these pump laser waves. These acoustic waves cause a corresponding stimulated Brillouin scattering once clockwise and once counter-clockwise. Thus two Brillouin lasers are obtained, of which one provides laser waves propagating clockwise in the fiber ring resonator and the other one providing laser waves propagating counter-clockwise in the fiber ring resonator. The frequencies of the stimulated Brillouin radiation in these Brillouin lasers are lower than the frequencies of the pump laser waves. They are also tuned to one resonance frequency of the fiber ring resonator. If there is an angular rate, also these resonance frequencies are shifted, whereby, also here, a beat frequency proportional to the angular rate is generated ("Applied Optics" Vol. 19 (1980), 1906-1908; "Optics Letters" Vol. 16 (1991), 229-231).
Also with such Brillouin ring lasers, there will be the insensivity due to "lock-in" in the range of small angular rates, as described above with reference to ring laser gyros.
It is known, in order to avoid the "lock-in", to operate the two Brillouin ring laser of the Brillouin ring laser gyro in two different modes, i.e. at two different resonance frequencies of the fiber ring resonator ("Optics Letter", Vol. 16 (1991), 230).
It is also known to excite the "clockwise" ring laser and the counter-clockwise ring laser of a Brillouin ring laser gyro with different pump lasers ("Optics Letter" Vol.16 (1991), 393-395). From this publication, an arrangement is also known, wherein pump laser supply fibers for the two pump lasers are formed by two ends of one single fiber. The fiber extends from one pump laser to the other one. In a median portion of the fiber, the fiber is coupled with the fiber ring resonator. In order to superpose the laser light beams from the oppositely directed Brillouin ring lasers, a separate waveguide network with a superposition directional coupler is provided, which is branched-off by couplers from the pump laser supply fibers.
By using two Brillouin ring lasers with substantially different frequencies (two-color ring laser gyros), influencing of one of the Brillouin ring lasers by the other one, and thereby "lock-in", is avoided. However also with two-color ring laser gyros, there are frequency fluctuations of the Brillouin ring lasers, which will deteriorate the measuring accuracy and inhibit the measurement of small angular rates such as of the angular rate of the earth.
EP 0 393 987 A2 discloses a ring resonator fiber optic gyro with a single detector on which the clockwise and counterclockwise beams interfere on leaving the beam. An angular rate causes a phase shift of the clockwise and counterclockwise beams and a corresponding interference signal at the detector. The output of the detector is used by a first and a second servo loop to compensate for optical path length changes and for rotation of the gyro, respectively. Both beams have the same frequency. There is no beat frequency. An optical isolator in front of the laser prevent the laser from being affected by the coupled-out resonator beams. EP 0 393 987 A2 shows no Brillouin ring laser.
EP 0 240 949 A2 shows a passive ring resonator angular rate sensor, wherein a pair of beams from a laser is injected clockwise and counter-clockwise, respectively, into a passive resonator. There are complex control loops to compensate for the change of optical path lengths, when an angular rate is experienced. These control loops provide a measure of this angular rate. Also EP 0 240 939 A2 does not show any Brillouin ring laser.
EP 0 516 332 A2 shows an electrically tunable fiber ring laser. This ring laser contains a resonance cavity in the form of a fiber ring. This resonance cavity contains a laser medium in the form of an erbium-doped fiber amplifier, which introduces optical gain into the fiber ring. The erbium-doped fiber amplifier is pumped by a pump laser in the form of a laser diode through a fiber and a coupler. The frequency of this laser can be tuned by a fiber Fabry-Perot. Principally, this is a classical ring laser gyroscope.
German patent application 27 20 256 discloses a method of measuring an inertial angular rate comprising the steps of providing, by means of two lasers, two monochromatic beams, which propagate in opposite directions in a passive ring resonator and of determining the frequency shifts of each of these two beams.
German patent application 42 18 905 describes a Brillouin ring laser with a fiber ring resonator. A pump laser generates cw and ccw acoustic waves in the fiber ring resonator. The cw and ccw Brillouin lightwaves generated in the fiber ring resonator are superposed at two detectors through a six-port coupler to cause interferences. Directional signals can be derived from the two detector signals by a direction detecting circuit.
EP 0 104 942 A2 shows a Brillouin ring laser with a fiber resonator formed from a loop of fiber optic material and a fiber optic directional coupler for optically closing the loop. An optical isolator consisting of a polarizer and a quarterwave plate is arranged in front of the laser. This optical isolator serves to prevent reflected light from entering the laser and interfering with its operation. A cw and a ccw ring laser is provided in the fiber ring resonator due to Brillouin scattering. The laser frequencies depend on the optical path lengths of the fiber ring resonator. If the fiber ring resonator is subjected to angular rate, there will be a beat frequency of the two laser frequencies. This beat frequency is effective at a detector and provides a measure of the angular rate.